The following known prior U.S. Patents are incorporated herein by reference: 4,360,226 to Takeuchi et al for Contact Printing Method and Apparatus; 4,054,383 to Somers et al for Jig and Process for Contact Printing; 4,007,987 to Sheets for Vacuum Contact Printing System and Process for Electronic Circuit Photomask Replication; 3,941,475 to Sheets for Optical Microcircuit Printing System; 3,834,815 to Greig for Photocopying Apparatus with Vacuum Means; and 3,658,417 to Lewis et al for Contact Printing Apparatus and Method.
In known contact printers of working masks, the system of a two part printing chamber with a vacuum generated thrust force is generally applied. Both parts of such a printing chamber are provided with identical clamping frames for the masks so that a clamping frame of the original forms with a respective part of the chamber a vacuum space for clamping the mask of the original. A clamping frame for a copy together with a second part of the chamber forms a similar space for clamping the mask of the copy (the working mask). Both masks are generally of the same size and usually of rectangular shape. The stable part of the chamber is provided at the bottom either with a collimating lens or with an opening for passage of light for the exposure. The bottom of the mobile part of the chamber is provided with a filter for observation of interference bands. After closing of both parts of the chamber with inserted masks for the original and for the copy, a central space is created between both parts, which, after connection to a source of vacuum, and, in case of airing of both external spaces, creates the required thrust force acting on both inserted masks. The masks contact, by their circumferential parts, packings which are maintained in place within recesses of the bodies of the clamping frames by means of thrust frames by clamping screws. A free part of the surface of the packings is left between the external circumference of the masks and the opening at the bottom of the recess of the clamping frame, enabling movements of the masks in order to secure their mutual correct contact in the course of copying.
However, simultaneously, an unwelcomed deflection of the extending surfaces of the packings in a direction towards the central space takes place, whereby tangential stresses are generated in the masks, which are distributed irregularly around the circumference of the masks, creating unacceptable faulty positions of the motive (image) of the original against the motive (image) of the copy. Said faults are minimized by application of some special materials for packings, the reinforcing insert of which has to show the same stiffness in two mutually perpendicular directions (perpendicular to two adjacent edges of masks). The packing material must furthermore prevent any sticking of the masks on the surface of the packings. Critical stress points of the packings are at the contact places of corners of masks with packings, where two mutually perpendicular directions of deflections cross. At these places, also changes of the stiffness due to fatigue of the material takes place and thus an increase of the position fault beyond the allowed limit. This fatigue of the packing material is experienced approximately after four months. Known packings usually comprise a rubber gasket reinforced by a textile fabric. Due to the high price and due to difficulties in securing similar packings, the operating costs of the whole arrangement are substantially increased.